"If they were called on their bluff they would lose the fight," says researcher zoologist Dr Robbie Wilson of the University of Queensland.

Most male crayfish are extremely aggressive and can fight over just about everything including females, food and shelter, says Wilson.

Their principal weapons are their two front claws, which are capable of grabbing and throwing the opponent around, ripping off a claw or even killing him.

"It's amazing to witness a fight. It can go on for as long as an hour," says Wilson.

But most crayfish disagreements are sensibly settled before it reaches this stage, by crayfish sizing each other up, says Wilson.

For example, in one ritual, a crayfish will lay down his claws while his opponent taps and rubs them.

After this, one crayfish usually backs off in the belief that they are no match for their opponent's apparent strength.

But, Wilson and co-author ecologist Dr Michael Angilletta of Arizona State University, Tempe have found that crayfish can bluff their opponents.

They studied the interaction of about 70 crayfish and tested the actual strength of their claws by measuring the force with which they pinched two pieces of metal together.

The researchers found that the size of a crayfish's claws does not reflect its actual strength. Some crayfish in their study displayed large claws but had very little muscle power.

"We call this dishonesty," says Wilson. "They were displaying to others that they were strong, high-quality males, but in actual fact they were cheating. They were actually weaker than what they were actually displaying."

Deceptive crustaceans

Unlike soft-skinned humans whose muscles are on display for everyone to see, crustaceans' hard shell, or exoskeleton, is particularly good at hiding how strong they are.

"It's like putting a shell around a human arm and you have no idea how strong the person is until they start punching you," says Wilson.

Given that 80 per cent of disagreements are settled at the sizing-up stage, this leaves a lot of room for deception.

As well as some large claws actually having very little strength due to a lack of muscle, some small claws can be stronger than larger ones.

To add to this complexity, the two front claws on a crayfish can differ in size and the larger claw may be stronger, weaker or the same strength as the smaller claw.

"It's an added layer of confusion for an opponent during a fight," says Wilson.

"It's very difficult for an opponent to actually know the animal's overall strength but also which is going to be the stronger claw."

Dangerous game

So, what do crayfish do with all of this?

In most cases, they will simply judge on the basis of claw size, says Wilson, but in about 25 per cent of cases, there will be bluffing involved.

He says the fact that it's potentially costly to call an opponent's bluff encourages the evolution of bluffing.

But the fact that it's also costly if you get your bluff called limits the evolution of deception.

"If you do get figured out, you get severely punished," he says. "It's a very dangerous game."